Continental Controls Needed to Fightback Against Tree
Diseases

Tighter controls on timber and plant movements into Europe are necessary to prevent further disastrous effects of plant diseases, a new study of the ash-dieback pathogen advises.

The call to action follows detailed investigations carried out
in British woodlands into the population makeup of the Hymenoscyphus
fraxineus fungus that causes ash
dieback.

While the findings bring some hope for the future of ash tree
populations in Great Britain and continental Europe, the authors
warn that further introductions of variants of the fungus from
its native East Asia must be prevented.

Professor James Brown from the John
Innes Centre, one of the authors
of the peer review paper in the journal Plant Pathology, said:
“What this study shows is that once the ash dieback fungus arrived
in Europe, it spread to Britain both by wind-borne spores and by
trade in plants. Other alien diseases could spread in the same
way. Because of this, disease control must operate on a European
scale. Above all, we should prevent diseased plants getting into
Europe.”

The research team from the John Innes Centre and Forest Research
carried out genetic tests on fungal samples found on ash leaves
and stems collected from infected woodlands in England and Wales.

Three of the woodland study sites in Norfolk, Suffolk and Kent
in the east of England contained ash populations believed to have
been infected by windborne fungal spores that travelled from diseased
woodlands in continental Europe.

Two further English sites, in Devon, Derbyshire and one in Carmarthenshire,
Wales, were selected because they were presumed be infected from
ash trees planted in the last 10-20 years from nursery stock from
continental Europe.

The aim of the research was to determine if the mode of arrival
of the fungus had an impact on the genetic diversity of fungal
populations and how they were adapting to local population of ash.

A range of lab and field techniques revealed that numerous infections
had established many strains of the pathogenic fungus at each location.
There was considerable genetic diversity within pathogen populations
in all the sites, regardless of mode of arrival.

The genetic diversity found in European populations of the fungus
had been transferred to all the British woodlands, whether they had
been introduced as windborne spores or on imported nursery stock.

This genetic diversity is, surprisingly, a positive sign for the
future of ash because it allows natural selection to operate within
populations of the fungus. The pathogen is expected to gradually
evolve over successive life cycles to adapt to the tree rather than
killing it.

“What we expect in the long run is that the ash and the fungi will
reach equilibrium - a kind of armed stand-off, and the fungus will
merge into the background as a parasite of only moderate importance,”
said Professor Brown.

But the study warns that this prospect is based upon the present
make-up of the population in Britain and continental Europe. The
danger is that new arrivals from East Asia of more genetically diverse
strains of the pathogen could be disastrous for European ash.

“It’s strongly suspected that ash dieback disease was imported by
timber movements from East Asia. Transport of plant material between
countries carries a real risk of spreading disease,” said Professor
Brown.

This study has implications for timber and plant trade within Europe,
“We have to be more rigorous about trade between European countries
once it is known that a disease is in the source country. It would
not stop spread of the disease but it would certainly slow it down,”
says Professor Brown,

Dr Elizabeth Orton, a post-doctoral scientist at the John Innes
Centre and lead author on the paper, recommends the creation of nurseries
containing trees with diverse resistance to the ash dieback pathogen.
Nurseries like these would allow the trees to breed together to produce
seed that can be distributed to bolster resistance throughout the
UK.